Knowledge of how most pathogenic microbes adapt to changing conditions within the host is limited. Advances in our understanding of the basic mechanisms of adaptation, gene regulation and virulence gene expression will provide new molecular targets for therapy to reduce the large medical burden imposed by Helicobacter pylori, which chronically colonizes over half of the world's human population and causes gastritis, ulcer disease, gastric carcinoma and mucosa-associated lymphoid tissue (MALT) lymphoma. We have previously shown that H. pylori regulates expression of key virulence factors and a small subset of regulatory proteins in response to environmental stress. We hypothesize that these regulatory factors modulate expression of genes that are crucial for adaptation to environmental stress and are likely to be essential for colonization and/or persistence of H. pyloriwithin the gastric environment. Herein, we propose detailed genetic and biochemical characterization of one of these factors, Fur. Fur is the only one of the regulators affected by both low pH and iron. Additionally, the classic paradigm of Fur regulation established by studies in other bacteria is considerably more complex in H. pylori', Fur regulates gene expression in both its iron-bound and apo forms. Our studies will define and characterize the iron- bound and apo-Fur regulons, and will seek to identify structural determinants of Fur that mediate each of these two modes of regulation. Finally, to expand our knowledge of virulence gene regulation in response to stress, we will identify additional genes involved in expression of the iron- regulated virulence genes cagA and vacA using reporter constructs and a near-saturating transposon library. These studies will fill a fundamentalgap in knowledge concerning theprocess of adaptation and regulation in H. pylori and should provide potential new therapeutic targets for H. pylori. Additionally, they willprovide novelinsight into the unique mechanisms of Fur-regulation utilized by this importantpathogen. Relevance to Public Health: H. pylori infects more than 50% of the worlds population and causes a range of diseases. Our studies will help to shed light on genes involved in the process of surviving in the human body and should provide novel targets for vaccine and therapeutic design.